Our Gemara on Amud Aleph discusses the Talmudic equivalent of the Good Samaritan Law. Namely, that even though ordinarily someone who makes a partial admission to a claim is Biblically obligated to make an oath (Shevuas Modeh Bemiktzas), if he is returning a lost object, the rabbis exempted him. 

To illustrate, ordinarily if Reuven accuses Shimon of owing him $100, and Shimon denies it, Shimon is Biblically exempt as there is no proof or grounds for the claim, and the burden of proof is on Reuven (there is a rabbinic obligation to make an oath at times in response to a denied claim called a shevuas hesis.). However, if Reuven accuses Shimon of owing him $100, and Shimon says, “I only owe you $50”, now Shimon must make an oath.  Logically speaking, we can say that Shimon’s partial admission gave Reuven’s claim some basis. The Gemara elaborates further in stating, the Torah anticipates human nature.  Really Shimon wanted to fully admit, but he didn’t have enough cash. He didn’t feel comfortable lying either, so he rationalizes, “I’ll partially admit to what I can pay, and then when I get the money I’ll come clean and pay off the balance.” Therefore to forestall this rationalization, the Torah obligates an oath upon Shimon to flush him out. Even Shimon, who contemplated “fibbing”, would not allow himself to transgress and lie under oath.

Having said all that, if someone returns a lost object and the owner says, “Wait a minute. There was $100 in my wallet!” But the Good Samaritan responds, “No, there was only $50.”, even though technically and Biblically he is obligated in an oath, the rabbis exempted him. This is because human nature being what it is, if we did not enact these protections, no one would want to return a lost object for fear of being embroiled in accusations.

However, if we reflect on this a bit more, the following inconsistency emerges. In one case, as we saw, the Torah itself makes allowances for human nature and introduces the oath of Modeh Bemiktzas. However, in the situation of the Good Samaritan returning a lost object, it is left to the rabbis to compensate and adjust for human nature?

We may infer the following. Though stemming from human psychology and therefore technically possible to resist, there are certain aspects of human nature that are such a given, that the Torah itself intervenes. Thus, the Torah accepts as a given, that a certain percentage of debtors will rationalize and hedge these claims through partial admission. Yet, in regard to the possibility of Good Samaritans refusing to return lost objects out of fear that they might be cornered into an oath, we must say the Torah was not convinced that every person returning a lost object will abscond to avoid accusations. Therefore, as apparent from rabbinic legal traditions, the Torah left it up to the rabbis throughout the generations to decide when and how to enact modifications, to work within each particular culture and the time.

However this answers the “what”, but not the “why”. Meaning, why is it more of a given that the person would rationalize lying than rationalize absconding? Perhaps there is a difference between the impulse to avoid payment versus the impulse to withhold being generous and returning a lost object. The impulse to avoid payment is stronger and more universal, so the Torah anticipates it without requiring rabbinic adjustment. Perhaps we can say the impulse for avoidance is in general stronger than the impulse to avoid being generous, as the person who owes money feels victimized and beleaguered by his creditor, and rationalizes his falsehood to buy time. However, the person who is returning a lost object likes to think of himself as generous and might overcome the urge to abscond, despite the risk of being ensnared in accusations and oaths.